The present invention relates to a method and apparatus for transporting mass and energy and for drying coatings on a substrate. More particularly, the present invention relates to substrate drying with magnetic particle orientation.
Drying coated substrates, such as webs, requires supplying energy to the coating and then removing the evaporated liquid. The liquid to be evaporated from the coating can be any liquid including solvents such as organic solvent systems and inorganic systems which include water-based solvent systems. Convection, conduction, radiation, and microwave energy are used to supply energy to coated webs. Applied convection or forced gas flow is used to remove the evaporated liquid. Applied convection is defined as convection produced by the input of power and caused intentionally. It excludes convection caused merely by web movement, natural convection, and other, unavoidable, forces. In some instances where the vapors are non-toxic, such as water evaporation, the vapor is removed by flashing off into the ambient atmosphere.
In conventional drying technology, large volumes of gas, inert or not, are required to remove evaporated liquid from the gas/liquid interface. These dryers require large spaces between the coated web being dried and the top of the drying enclosure to accommodate the large gas flows. Drying is governed at the gas/liquid interface by diffusion, convection, boundary layer air from the moving web and impinging air streams, vapor concentrations, and liquid to vapor change-of-state convection, among other factors. These phenomena occur immediately above the coated web, typically within 15 cm of the surface. Because conventional dryers have a large space above the coated web, and they can only control the average velocity and temperature of the bulk gas stream, they have limited ability to control these phenomena near the gas/liquid interface.
For organic solvent systems, the vapor concentrations in these bulk gas streams are kept low, typically 1-2%, to remain below the flammable limits for the vapor/gas mixture. These large gas flows are intended to remove the evaporated liquid from the process. The expense to enclose, heat, pressurize, and control these gas flows is a major part of the dryer cost. It would be advantageous to eliminate the need for these large gas flows.
These gas streams can be directed to condensation systems to separate the vapors before exhausting, using a large heat exchangers or chilled rolls with wiping blades. These condensation systems are located relatively far from the coated web in the bulk gas flow stream. Due to the low vapor concentration in this gas stream, these systems are large, expensive, and must operate at low temperatures.
It would be advantageous to locate the condensation systems close to the coated substrate where the vapor concentrations are high. However, conventional heat exchangers would drain the condensed liquid by gravity back onto the coating surface and affect product quality unless they were tilted or had a collection pan. If they had a collection pan they would be isolated from the high concentration web surface. If they were tilted dripping would probably still be a problem. Also, conventional heat exchangers are not planar to follow the web path and control the drying conditions.
U.S. Pat. No. 4,365,423 describes a drying system which uses a foraminous surface above the web being dried to shield the coating from turbulence produced by the large gas flows to prevent mottle. However, this system does not eliminate applied convection, requires using secondary, low efficiency solvent recovery, and has reduced drying rates. Also, because of the reduced drying rates, this patent teaches using this shield for only 5-25% of the dryer length.
German Offenlegungeschrift No. 4009797 describes a solvent recovery system located within a drying enclosure to remove evaporated liquid. A chilled roll with a scraping blade is placed above the web surface and removes the vapors in liquid form. No applied convection removes the evaporated liquid. However, the roll is only in the high vapor concentration near the surface for a short section of the dryer length. This does not provide optimal control of the conditions at the gas/liquid interface. In fact as the roll rotates it can create turbulence near the web surface. Also, this system can not adapt its shape to the series of planar surfaces of the coated web as it travels through the dryer. Therefore, the system can not operate with a small, planar gap to control drying conditions and can not achieve optimum condensing efficiency.
U.K. patent No. 1 401 041 describes a solvent recovery system that operates without the large gas flows required for conventional drying by using heating and condensing plates near the coated substrate. The solvent condenses on the condensing plate and then condensed liquid drains by gravity to a collection device. This apparatus uses only gravity to remove the liquid from the condensing surface. Accordingly, the condensing surface can not be located above the coated substrate since gravity will carry the condensed liquid back onto the coated substrate. In the drawings and discussion (page 3, lines 89-92) the condensing surface is described as vertical or with the coated substrate, coated side facing down, above the condensing surface. Applying a coating to the bottom side of the substrate or inverting the substrate after application of the coating is not the preferred method in industry. Coating in an inverted position and inverting a coated substrate before drying can create coating defects. These limitations greatly reduced the flexibility of the method and entail significant costs to adapt it to standard manufacturing methods. This requirement for vertical or inverted drying is very likely the reason this method has not been adopted or discussed in the industry.
U.K. patent No. 1 401 041 also describes, on page 2 line 126 to page 3 line 20, the problems of this method with growth of the liquid film layer on the condensing surface and droplet formation. Because xe2x80x9cthe resulting liquid film 14 may increase in thickness towards the lower end of the condenser,xe2x80x9d the length of the condensing surface is limited by the buildup and stability of this film layer. Limiting the length of the condensing surface will limit the dryer length or require exiting the drying system with the coating not dried. This has the undesirable effect of losing some of the solvent vapors to the atmosphere, losing control of the drying phenomena, and creating defects. Another limitation is that the distance of the condensing surface from the coated substrate xe2x80x9ccan hardly fall below about 5 millimetersxe2x80x9d to prevent contacting the condensing liquid film with the substrate, and to prevent droplets from contacting the substrate.
The limitations of this system to vertical or inverted drying, limits in the length of the dryer, and the inability to operate at desired distances from the coated substrate render it inadequate to achieve the desired drying benefits.
Dryers used to dry magnetic coatings also are known. Known systems place a magnetic field generator inside the dryer to orient the magnetic particles within the coating being dried. However, conventional orienting devices inside the dryer disrupt the air flow and impair drying, causing the surface of the product to roughen. As the particles leave a conventional orienting device in the early stages of drying, any components of the magnetic field which are not in the plane of the coating will reorient the particles in a non-preferred direction.
There is a need for a system for drying coated substrates while orienting magnetic particles in the coating, which provides improved control of the conditions near the gas/liquid interface and in which the orientation process does not interfere with the drying. There is also a need for a system that can operate with small gaps adjacent the substrate.
The invention is a method and apparatus of drying a coated substrate that includes magnetic particles. A condensing surface is spaced from the substrate and substantially corresponds to the path of the substrate in the longitudinal direction. This creates a longitudinal gap between the substrate and the condensing surface. Liquid is evaporated from the substrate to create a vapor and the vapor is transported to the condensing surface without requiring applied convection. The vapor is condensed on the condensing surface to create a condensate, and the condensate is removed from the condensing surface. Removal is performed, using more than gravity, without allowing non-uniformities of the condensate film to occur. The magnetic particles are oriented on the coated substrate by subjecting the coated substrate to a magnetic field created at a location outside of the space between the condensing surface and the coated substrate. The magnetic field can be created by a magnetic field generator.
The magnetic field can be created at various locations: a location separated from the substrate by the condensing surface, a location separated from the condensing surface by the substrate, a location surrounding the substrate, and any combination of these locations.
The magnetic particles can be oriented at the beginning of the evaporating step and holding the magnetic particles in a preferred direction during the evaporating, condensing, transporting, and removing steps.